1. Field of the Invention
A driver circuit of the present invention relates to an output driver circuit the output signal of which is connected to an input of a device in the next stage via a member having a parasitic inductance such as a wire and performs current drive or voltage drive and, more specifically, to a drive circuit in which the effect of the parasitic inductance that varies according to applied material and packaging can be reduced, and good output matching and wide band characteristics can be maintained.
2. Description of the Related Art
Recently, as the Internet becomes widespread, transmission rates required for an IC for optical communication is ever-increasing into 10 Gb/s to 40 Gb/s. In order to realize the speeding up of the IC, it is necessary that high speed response characteristics of transistors constituting the IC are improved, and characteristic deterioration such as band limiting at the time of mounting is suppressed. Especially, in an LC circuit constituted by a parasitic capacitance (a pad capacitance, an output capacitance of a transistor, etc.) of the output end of an IC or a parasitic inductance such as a wire connected to the input of a device in the next stage, band limiting and output mismatching becomes a factor of characteristic deterioration.
In order to solve the problem, conventionally, a filter as shown in FIG. 19.6.2 of “40 Gb/s 2:1 Multiplexer and 1:2 Demultiplexer in 120 nm CMOS,” ISCCC Digest of Technical Papers, pp. 344–345, 2003 is constituted at the output end of the IC.
A conventional configuration example of a driver circuit having the filter of the above document is shown in FIG. 11. In FIG. 11, an IC chip 1 as the driver circuit has a pair of differential transistors Q1, Q2, a constant current source I1 connected between emitter terminals of the pair of differential transistors Q1, Q2 and a negative voltage terminal within the IC for setting current driving in the pair of differential transistors Q1, Q2, emitter follower transistors Q3, Q4 for inputting output signals drawn from the emitter terminals of themselves to base terminals of the pair of differential transistors Q1, Q2, constant current sources I2, I3 connected between the emitter terminals of the emitter follower transistors Q3, Q4 and the negative voltage terminals within the IC, respectively, for setting current flowing in the emitter follower transistors Q3, Q4, terminating resistors R1, R2 for IC internal matching connected to collector output terminals of the pair of differential transistors Q1, Q2, inductors L1, L2 connected between the collector outputs of the pair of differential transistors Q1, Q2 and a positive voltage terminal within the IC via the terminating resistors R1, R2, respectively, bonding pads P1, P2 respectively provided on differential output parts of the IC chip 1, inductors L3, L4 inserted between the collector output terminals of the pair of differential transistors Q1, Q2 and the bonding pads P1, P2, respectively, a bonding pad P5 connected to the positive voltage within the IC, and a bonding pad P6 connected to the negative voltage within the IC.
A wire L9 connects the pad P5 and a positive voltage source on the mounting substrate, a wire L10 connects the pad P5 and the negative voltage source on the mounting substrate.
A light output part 2 driven by the IC chip 1 includes an optical modulator D and a resistor R3 connected in parallel with the modulator, and an anode of the optical modulator D is connected to the pad P5 through a wire L5 and a cathode of the optical modulator D is connected to a positive voltage source. Further, a terminating resistor R4 is connected between the pad P2 and the positive voltage source via a wire L6.
In the driver circuit in FIG. 11, the wideband characteristics of the IC are realized by the effects of the inductors L1, L2 serially connected to the terminating resistors R1, R2, respectively, for inductor peaking, and the inductors L3, L4 connecting the respective collector outputs of the pair of differential transistors Q1, Q2 and the IC output pads P1, P2. Further, the filter is constituted by the on-chip inductors (L1 to L4), output capacitances of the pair of differential transistors Q1, Q2, the pad capacitances of P1, P2, and the bonding wires L5, L6 and, by optimizing the L value, the cutoff frequency of the filter itself can be made sufficiently higher so that the cutoff frequency may not become a factor of the band limiting of the entire IC. Further, characteristic impedance of the filter can be set to desired values by the optimization of the L value. The improvement in bands and good output matching characteristics have been realized by the output circuit having such a filter.
Moreover, in “20 Gb/s transimpedance preamplifier and modulator driver in SiGe bipolar technology”, IEE Electron Lett. Vol. 32, No. 13, pp. 1136 to 1137, 19, Jun., 1997), rising/falling time of an output waveform is improved by inserting a bonding wire in serial with a terminating resistor.
In the driver circuit having the filter in the conventional form, inductances of the wires L5, L6 are required to be specified at the time of design. However, in a driver for driving a laser diode or an optical modulator, for example, there are some cases where used packages are different according to their application, the driver is bare chip mounted for miniaturization of a module and a transmission unit, or an IC and an optical element are spaced and connected by a long wire by intention so that the effect of heat generation of the IC may not adversely affect the characteristics of the optical element. On this account, it has become difficult to uniquely determine inductances of the wires L5, L6 in advance.
Therefore, despite the fact that the filter is provided, there has been a problem that band deterioration and output mismatching can not be suppressed sufficiently due to the effect of inductances that vary according to packaging. Furthermore, in the case where the filter is constituted in a driver circuit having the conventional form in which the signal from the emitter follower is amplified, there has been a problem that, as described in detail below, the characteristics of the emitter follower affect the output impedance of the driver circuit and output capacitances of the pair of differential transistors Q1, Q2 do not appear to be ideal capacitances; thereby, good filter characteristics can not be obtained.
The effect on the output impedance of the differential amplifier when output impedance of the differential amplifier, the output impedance of the emitter follower, and the emitter follower are connected to the input will be described below. Basic equivalent circuits of the differential amplifier are shown in FIGS. 12a to 12c. FIG. 12a is a circuit diagram of the differential amplifier including the transistors Q1, Q2 and the terminating resistors R1, R2. When a differential signal is input to the differential amplifier in FIG. 12a, the common emitter point becomes a virtual grounded point, and the differential amplifier can be replaced by a single-ended equivalent circuit. Therefore, the small-signal equivalent circuit is shown by FIG. 12b (see “Analysis and Design of Analog Integrated Circuits-Fourth Edition-”).
In FIG. 12b, RS denotes an output impedance of an input signal source, rb denotes a base resistor, rπ denotes an input resistor, Cπ denotes a base-emitter capacitance, Cμ denotes a base-collector capacitance, gm denotes a transconductance, RL denotes a resistance value of the terminating resistor R1 or R2, vi denotes a voltage of the signal source, v1 denotes a voltage applied to both ends of the resistor rπ, and vo denotes an output voltage drawn from both ends of the resistor RL.
FIG. 12c shows an output impedance equivalent circuit obtained from the equivalent circuit in FIG. 12b. As shown in FIG. 12c, the output impedance of the transistor Q1 or Q2 is represented by two CR series circuits connected in parallel. Of these two CR series circuits, because the magnitude of a capacitance proportional to gm varies according to the condition of the collector current of the transistor Q1 or Q2, when the collector current is large, the impedance of the CR circuit constituted by the capacitance proportional to gm becomes small and dominant; however, because, in a condition in which the transistor is off, the capacitance proportional to gm becomes smaller than Cμ as gm becomes smaller, in turn, the impedance of the CR circuit constituted by Cμ becomes dominant. Consequently, the output capacitance of the transistor Q1 or Q2 is estimated as C of the either dominant CR circuit.
FIG. 13 shows S22 (a voltage reflection coefficient indicative of the relationship between the input voltage and the reflection voltage) as S parameter of the differential amplifier in a condition in which the collector current is relatively larger plotted on a Smith chart. It is also seen from FIG. 13 that the output impedance of the differential amplifier is represented by a parallel circuit of the load resistor RL and the CR series circuit, that is, the output impedance of the transistor is represented by the CR series circuit. Therefore, in the case of constituting the filter, the filter is designed in consideration of the value of C in the CR series circuit estimated from the Smith chart.
Next, the output impedance of the emitter follower will be described. Basic equivalent circuits of the emitter follower are shown in FIGS. 14a to 14c. FIG. 14a is a circuit diagram of the emitter follower circuit including the transistor Q3 or Q4. Further, the small-signal equivalent circuit of the emitter follower circuit is shown by FIG. 14b. Since the output impedance equivalent circuit obtained from the equivalent circuit performs inductive operation in the case where I2 and I3 are equal to or more than several hundred micro amperes and 1/gm=(RS+rb) in a high-speed circuit, the output impedance equivalent circuit is represented as shown in FIG. 14c (see “Analysis and Design of Analog Integrated Circuits-Fourth Edition-”).
FIG. 15 shows S22 of the emitter follower plotted on the Smith chart. It is also seen from FIG. 15 that the output impedance of the emitter follower is represented with inductivity.
Thus, when the output impedance of the emitter follower shows inductivity and the emitter follower is connected to the input part of the differential amplifier, the effect on the output impedance of the differential amplifier is shown in FIGS. 16a, 16b and 17. FIG. 16a shows an equivalent circuit of the differential amplifier when the emitter follower is connected to the input of the differential amplifier, and the output impedance RS of the input signal source in FIG. 12b is replaced by the output impedance Zout_ef of the emitter follower.
FIG. 16b shows an output impedance equivalent circuit obtained by the equivalent circuit shown by FIG. 16a. Since a resistor Rosc inversely proportional to square of frequency ω appears in the equivalent circuit, and a parallel resonant circuit of Cπ and L is formed, the output impedance of the transistor Q1 or Q2 can not be regarded as a simple CR series circuit.
FIG. 17 shows S22 of the differential amplifier when the emitter follower is connected to the input of the differential amplifier. Compared to S22 shown in FIG. 13, the graph has a form bulging toward outside and this reflects the effect of the resistor Rosc. Thus, the inductive operation of the output of the emitter follower affects the output impedance of the differential amplifier and, as a result, the operation also affects the filter characteristics of the driver circuit output having output capacitances of the pair of differential transistors Q1, Q2 as component elements; thereby, it becomes difficult to obtain a desired inductor peaking amount or output matching and that causes band deterioration or output mismatching.